It is well known that the nature and content of the abrasive grains in a bonded abrasive article have significant influence on the grinding performance of such articles. The term "nature", as herein used, refers to chemical composition, morphology, shape, and size for both the individual abrasive grain and any microcystalline components thereof. For example, each grit of the sintered sol gel alumina abrasive grains of Cottringer, et al, (U.S. Pat. No. 4,623,364), is composed of an assemblage of alpha alumina crystallites each no larger than 0.4 microns. The sol gel alumina particles of Cottringer are generally described as "seeded".
Other sol-gel alumina abrasive are described in, for example, U.S. Pat. No. 4,314,827 (Leitheiser et al) and U.S. Pat. No. 4,744,802 (Schwabel). Leitheiser et al describes unseeded and Schwabel seeded sol gel alumina abrasives.
It is found that vitreous bonded articles prepared from sol gel alumina abrasive grits may outperform identical articles prepared from conventional fused alumina abrasive by as much as 600% or more in wheel life and in the ratio of metal removed to wheel wear, while consuming less power and removing metal at a higher rate. However, in organic bonded abrasive articles, sintered sol gel abrasives, while superior to normal fused alumina abrasives, have hitherto not shown the dramatic improvement seen in vitreous bonded articles.
At high grinding pressures, organic bonded abrasive articles wear too rapidly to take advantage of the superiority of the individual sol gel alumina grits and at low force levels not enough pressure is exerted on individual grains to microfracture for self-sharpening.
It is well known in the art that dilution of the abrasive grit portion of bonded abrasive articles with softer more friable particles will increase the cutting rate. Hartmann, in U.S. Pat. No. 1,830,757, teaches a self-sharpening wheel in which friable particles of calcined clay, porous alumina, magnesia or glass, all softer than the abrasive grits, are incorporated to break out during grinding leaving an open, free-cutting grinding face. Similarly, Robie, in U.S. Pat. No. 2,806,772 teaches the use of thin walled balloons, resin micro balloons, porous clay pellets, and other friable particles to provide pore support and increase freedom of cut. Sandmeyer, in U.S. Pat. No. 2,986,850, recognized the unique ability of hollow alumina spheres to serve as both abrasive grit and pore spacer in bonded abrasive articles. Grinding wheels of 100% alumina bubbles, made according to the Sandmeyer patent, wear very rapidly and are only suitable for grinding soft materials such as cork, rubber, fiber board, etc.. Fukada, in U.S. Pat. No. 4,226,602, incorporates pore-forming blowing agents with or without alumina bubbles in a continuously-formed resin bonded abrasive article of improved cutting ability. Zimmer, in South African Disclosure 82/0339, describes resin bonded abrasive articles of less than 14% interconnected porosity containing silane-treated porous support material, including alumina bubbles, and multicellular glass modules. The function of the porous materials in this instance was to prevent slumping during cure and reduce infiltration of water during wet grinding.
Bloecher, U.S. Pat. No. 4,799,939, teaches forming abrasive aggregates particles comprising abrasive particles and glass micro balloons in a phenolic resin bond. These particulate aggregates can then be used to manufacture abrasive products.
Other methods of introducing pores into organic bonded abrasive articles are well known, as exemplified by Pohl, in U.S. Pat. No. 1,986,850 in which uniformly distributed hollow spaces are produced in abrasive bodies by gas evolution and use of soluble or sublimable spheres.
This increase in cutting rate is, however, usually accompanied by a significant increase in the rate at which the abrasive article is worn down. This ratio of the volume of work material removed to volume of abrasive article worn away is known as the "G-ratio". Generally, high G-ratios are clearly preferred but have not hitherto been obtained with conventional abrasive particles.
It has been found that the substitution of friable filler particles such as bubble alumina spheres or softer, more friable, particles for a portion of the sintered sol gel alumina grains not only increases the cutting rate, but decreases the wheel wear for unexpectedly high G-ratios. This is a most unexpected result and runs counter to the teachings of the prior art.
It is, therefore, an object of the present invention to provide an abrasive article in which sintered sol gel alumina abrasive particles are resin bonded and yet provide increased cutting rates with decreased wheel wear.
Another object of the invention is to provide an abrasive article offering the advantages of sintered sol gel alumina abrasive particles without unacceptable wheel wear.
A further object of the invention is to provide an abrasive article combining excellent cutting rate and acceptable wheel wear characteristics with relatively inexpensive material cost.
Another objective of the invention is to provide an abrasive article which, for a given grinding rate, uses less power and is less likely to damage a workpiece metallurgically.
With these and other objectives in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the appended hereto.